Retractable measuring tapes and other devices for extracting and recoiling a coilable member have been on the market for many years. Commonly, these devices have evolved into a basic design which has a number of problems. The basic design is typically shown by U.S. Pat. No. 3,318,550 which includes a slotted center post affixed to the side of the housing with one end of a clock type torsion spring mounted in the post slot. A drum or cylinder is provided for rotation about the center post with the torsion spring within the drum and a measuring tape supported about the periphery of the drum. In the common design, the outer end of the torsion spring is usually attached to the inner end of the tape and may pass through a slot in the drum. The outer end of the tape is commonly provided with a hook member which is used to hold the end of the tape during measuring, and upon retraction, abuts the tape opening to prevent the end of the tape from being retracted into the housing.
In order to assure complete retraction, the torsion spring is pre-tensioned, that is, the spring is placed in tension even upon the tape being in a fully retracted condition within the housing. The accepted approach to pre-tensioning the torsion spring is to physically rotate the reel or drum supporting the spring and tape about the center post before the housing is assembled. The device may include a stop mechanism to retain the pretensioning of the spring, as described in U.S. Pat. No. 3,007,653.
As the tape is extended from the housing in the basic design, the tape turns the drum or cylinder and directly or indirectly winds the torsion spring to store energy therein needed for retraction. As a clock type torsion spring has a progressive spring rate, the force needed for tape extraction increases as the tape is withdrawn and thus provides maximum acceleration to the retracting tape when the tape is released from being extended. The continuous, although decreasing, retracting force causes considerable linear and rotational momentum to the retracting tape.
In the tape design heretofore described, breakage of the hook, either by misuse or through impact at the housing, which commonly occurs if the lineal and rotational momentum imparted to the tape upon retraction is not dissipated before the hook reaches the housing, allows the tape end to enter the housing resulting in elimination of the pretension of the spring and/or causing damage thereto. Repair of the tape, and frequently of the spring, must be made by opening the housing and handling the tape and spring to connect a new tape which must be manually wound and the spring pre-tensioned. Repair in this manner can be dangerous to the novice. Generally the damaged device is discarded rather than repaired.